If you put Armsmaster in a regular prison he could get up to shenanigans with relative ease. Would he? Up to your interpretation.
Assuming he really wanted out...
Melt end of toothbrush via. sustained friction, lighter, or by strategically breaking window to refract light. Get it soft, then press into one of the screws that holds the cage against the ceiling, where a grid of bars typically protect the light fixture. Let it harden again. Unscrew cage/mount for the light fixture. Dismantle bulb for filaments, phosphor, cathodes, glass, caps, etc. Reconstruct bulb with less components. If necessary, make a polite complaint that the light in the cell isn't working properly, have it swapped out, then repeat the process for more components. Chances are good people won't do a thorough check of the bulb and realize you've rigged it to be an old-fashioned incandescent light rather than a gas-discharge bulb. It still works, it just doesn't work well.
Obtain, steal, barter for, or smuggle a 2B pencil and metal components (coins, paperclips, staples). Battery would be a plus. Old guy with a hearing aid battery, tv remote, steal a phone and take the battery from there. Failing that, design one. Go tinkermode for ultra-fine etching of circuits via. pencil on paper and engineering of simple robot.
Robot that can fit in the palm of his hand is then used to find objects meeting certain crude criteria (within X, Y, Z, dimensions, bright, dark). With simple components gathered over days, advance robot. Can even do the circuit writing on paper in plain view - just play the part of the crazy tinker. Act obsessive about it. He can get them talking about whether not letting him have a workshop is detrimental to his mental health.
All the while, he's very sane and he's very focused. What he can't hide or have his robot hide, he places inside his skin, making incisions and sliding things under.
Is he going to build a nano-thorn Halberd? No. But two months into his sentence, that is a man who could be taking out guards and making it look like he's the one that's in control of the system, while he walks, not runs, through the various checkpoints and barricades. The guards end up being the ones that are flustered and trying to head him off, communications and cameras down.